| A trait was implemented on another which already automatically implemented it. | |
| Erroneous code examples: | |
| ```compile_fail,E0371 | |
| trait Foo { fn foo(&self) { } } | |
| trait Bar: Foo { } | |
| trait Baz: Bar { } | |
| impl Bar for Baz { } // error, `Baz` implements `Bar` by definition | |
| impl Foo for Baz { } // error, `Baz` implements `Bar` which implements `Foo` | |
| impl Baz for Baz { } // error, `Baz` (trivially) implements `Baz` | |
| impl Baz for Bar { } // Note: This is OK | |
| ``` | |
| When `Trait2` is a subtrait of `Trait1` (for example, when `Trait2` has a | |
| definition like `trait Trait2: Trait1 { ... }`), it is not allowed to implement | |
| `Trait1` for `Trait2`. This is because `Trait2` already implements `Trait1` by | |
| definition, so it is not useful to do this. |